A tactical guide to the infinite realm of science. Although the world of science would take eternity to explore, Professor Quibb attempts to scrape the edge of this Universe. This blog helps you to understand particular topics under the more general categories: cosmology, mathematics, quantum physics, meteorology and others. Join me on my trek across the untraversed lands of the unknown.

Sunday, March 5, 2017

Beginning in the 1990s, advances in astronomy allowed the detection of many extrasolar planets, adding thousands of the number known within two decades. However, apart from the reclassification of Pluto as a dwarf planet in 2006, the population of true planets in our Solar System did not change. Many, many other smaller objects were discovered, though.

Many of these smaller objects lay within the asteroid belt between Mars and Jupiter, or in the Kuiper Belt, just beyond Neptune's orbit. Eris, Haumea, and Makemake are other dwarf planets whose perihelia (closest approaches to the Sun) bring them within the Kuiper Belt, 30 to 50 astronomical units (AU) from the Sun. However, an unusual object was discovered in 2003 whose orbital properties were quite different.

The object was later named Sedna and measures a little less than half the diameter of Pluto. Though the best images of it by telescopes are only a few pixels wide, it is clearly of a reddish color, nearly as red as Mars. The perihelion of this object was, at the time, the largest known in the Solar System, at 76 AU. However, it also has an extremely elongated orbit, bringing it to an aphelion (farthest point) of 936 AU! This orbit is shown in red above, compared to the orbits of the outer planets and Pluto (in pink). About a decade later, another object, provisionally designated 2012 VP113, was discovered with comparable orbital parameters, except with a slightly farther perihelion of 80 AU and an aphelion of 438 AU. The scarcity of known objects of this type is not only a consequence of their distance, however.

This scatterplot, published in a paper by astronomers Chadwick A. Trujillo and Scott S. Shephard, shows the perihelia and eccentricities (a measure of the "elongatedness" of an elliptical orbit; a perfect circle has an eccentricity of 0) of various objects outside Neptune's orbit. Curiously, there is a clear drop-off at around 50 AU, with only a few known objects beyond. Notably, there is also a gap between 55 and 75 AU. This gap is not only an artifact of our telescopes being insufficiently powerful: Sedna and 2012 VP113 were detected farther out, so if there were objects in this gap they should have been easier to find. The high eccentricity of Sedna and 2012 VP113, as well as the existence of this gap, aroused suspicion that a massive object may have gravitationally perturbed the trajectories of objects in this region, illustrated in the image below.

The same paper indicated another unusual feature of the population of these farthest known objects.

The horizontal direction indicates the semi-major axis of each object (yet another measure of the size of an orbit; however, it is closely related to the two discussed previously: it is simply the average of the perihelion and the aphelion). The vertical variable on the scatterplot is the argument of perihelion, which is simply the angular position around the orbit of the orbit's perihelion (relative to where it crosses the plane of the Solar System). All known objects whose semi-major axes exceed 150 AU have arguments of perihelion all clustered roughly around 0°. In the eight-planet Solar System model, this should not be the case: gravitational perturbations from the gas giants would randomize the arguments of perihelion over millions of years. However, a large planetary body orbiting well beyond the known planets could constrain the arguments of perihelion. This led to the hypothesis of a new planet, nicknamed Planet Nine.

The above image shows the orbits of many of the same objects represented by dots to the right of the black line in the scatterplot. Note how in addition to the clustering trend noted above, the perihelia are also all on the same side of the Sun. The figure also shows where Planet Nine would possibly orbit given the positioning of those objects. The story of the Planet Nine hypothesis continues in the next post.

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Spacecraft Update

As of February 2018...

New Horizons: Launched in January 2006, the probe successfully flew by Pluto on July 14, 2015. It has now adjusted its trajectory for an additional encounter with a Kuiper Belt object 2014 MU69 on Jan 1, 2019. For more information about the New Horizons mission, see the main post, New Horizons.

Kepler: Launched March 7, 2009, the Kepler space telescope has discovered over 2300 new exoplanets! Though a malfunction in May 2013 seemed to end the data gathering mission, an ingenious new method of orientation allowed for a new mission, known as K2, to begin in 2014! For more information on the Kepler mission and the latest results, see the main post, Kepler.

Dawn: Launched in 2007, the probe visited and departed the asteroid Vesta and is now orbiting the dwarf planet Ceres! It has discovered organic materials on Ceres' surface and much more. For more information, see the main post, Dawn.

Juno: Launched on August 5, 2011, Juno's mission is to eventually assume a polar orbit of Jupiter and study its magnetic field, as well as its internal structure. The probe entered a polar orbit around Jupiter on July 4, 2016. For more information, see the main post, Juno.

Mars Science Laboratory: This mission's primary payload is a rover, Curiosity, by far the largest rover to date. Since it landed on Mars in 2012, this mission has analyzed the red planet with more than 5 times the scientific equipment of any of its predecessors. The rover has discovered, among other things, the existence of liquid water on Mars and compelling evidence that Mars could have supported life in the past. For more information, see the main post, Mars Science Laboratory.

MAVEN: Launched on November 18, 2013, MAVEN is a Martian orbiter which arrived at Mars on in 2014. Its mission is to investigate the Martian atmosphere and its interaction with solar wind. These data should provide precise evidence as to when and how Mars lost its atmosphere, and give further clues into whether it could have supported life billions of years ago. For more information, see the main post, MAVEN.

ExoMars: ExoMars is a mission to investigate possible traces of life on the planet Mars. The mission includes two launches: one in 2016 and one in 2020, with the first delivering an orbiter and a lander to Mars and the second the ExoMars rover. The first launch took place on March 14, 2016. For more information, see the main post, ExoMars.

OSIRIS-REx: OSIRIS-REx is a sample return mission to the asteroid 101955 Bennu. Launched on September 8, 2016, it will reach its destination in in December 2018. For more information, see the main post, OSIRIS-REx.